Optical measurement techniques are in wide use today in many industries. In addition to not requiring physical contact, such techniques may produce very accurate measurements. However, as with almost all tools, it is desirable to still further improve upon and develop new optical measurement techniques.
FIG. 1 (background art) is schematic block diagram depicting the conventional optical triangulation method of position detection. In this technique, a light source and a detector are positioned in a fixed relationship. A measurement target is placed in the path of a light beam produced by the light source, and this the light source, the detector, and the target form a triangle. The light beam shines on the target and is reflected back by the target to the detector. The distribution of reflected intensity on the detector, is measured and the angle of the target is calculated. Since the distance between the light source and the detector is fixed and the light beam direction is also fixed, the position of the target can then be calculated.
Unfortunately this simple triangulation technique suffers from a number of disadvantages. The light source, the detector, and the direction of motion of the measurement target (+/-Y direction) all have to be in the same plane. This means that the triangulation technique here can only measure in one degree of freedom, and this approach cannot be used to tell whether the target is moving longitudinally or laterally (+/-X or Z directions).
Of particular present interest, the triangulation technique does not address variations in the light used for measurement. Stabilizing the intensity from the light source or compensating for fluxuations in the light are not provided for here, and either or both may be desirable when high measurement precision is needed. The light reaching the detector may also be effected by conditions elsewhere than at its source. For example, target reflectivity may vary, particularly if different areas of a target are used for reflection at different points in measurement. Various factors can also effect the light path itself, rather than its triangle endpoints, such as air turbulence, the presence of particulate matter, etc.
These light variation factors also exacerbate other inherent problems with the triangulation technique. The level of sophistication in calculation required to determine the target position is high, and this also limits triangulation technique based system response times.
Lasers are widely used today in optical measurement, and in FIG. 1 a laser diode is depicted as being the light source. Although other types of lasers and even other light sources entirely may be used, laser diodes are becoming very popular for remote sensing due to their low cost, small physical dimensions, and high sensitivity. A problem with light sources and a particularly acute one for laser diodes is the effect of power variation on the stability of light beam intensity. The usual manner to address this problem is to design power stabilizing circuitry, but this may be unduly expensive in some cases and simply insufficient in others.
Mere light source power stabilization may also be ineffective, even with very closely controlled power stabilization. Laser diodes very in sensitivity based on the environmental temperature. Thus it often becomes necessary to build a housing to totally insulate them. This process is commonly done in research laboratories, but it is often not practical in an industrial environment.
Light source stabilization may be important but, as experienced engineers well know, merely applying correction does not mean that one can rely on it to occur in the manner and to the extent desired. Thus, even when stabilizing techniques are employed, it is highly desirable to also be able to verify their effectiveness.
In FIG. 1 a position sensitive type detector is depicted, such as a simple photo diode, but other detectors types may also be used, such as bi-cell detectors, quadrant detectors, and photodiode detector arrays. The detector used for optical measurement is usually the most stable and trustable component, and a detailed discussion of detectors is not germane to the present invention. Rather, our concern here is improving the art of optical measurement in the stages before the detector and to effectively verify that improvement.
FIG. 1 shows only the conventional triangulation method, wherein a light beam travels one path to the target and another from it to the detector. This geometry is useful in many situations, but not in all. For example, it may desirable to employ reflection but at normal angles, such that a light beam travels substantially the same path to and from the target to the detector. Or to even employ a linear arrangement, wherein the source, target, and detector form a line.
Accordingly, new and improved techniques of optical measurement are highly desirable and should be promptly and well received.